Spectroscopic and electrochemical properties of new mixed-ligand orthometalated rhodium(III) complexes

Abstract

The electrochemical behavior and the absorption spectra, emission spectra, and luminescent lifetimes of the mixed-ligand complexes Rh(phpy)2phen+, Rh(thpy)2phen+, Rh(phpy)2biq+, and Rh(thpy)2biq+ (where phen is 1,10-phenanthroline, biq is 2,2'-biquinoline, and phpy" and thpy" are the ortho-C-deprotonated forms of 2-phenylpyridine and 2-(2-thienyl)pyridine) have been studied. The results obtained have been compared with those previously available for the analogous Rh(phpy)2bpy+ and Rh(thpy)2bpy+ complexes (bpy = 2,2'-bipyridine), the [Rh(phpy)2Cl]2 dimer, the homoleptic Rh(phen)33+ species, and the orthometalated Pt(phpy)2-(CH2C1)C1 and Pt(thpy)2(CH2Cl)Cl complexes. The results obtained indicate that, as reported previously by Ohsawa et al., the NN-coordinating ligands are easier to reduce than the NC-orthometalating ligands. The absorption spectra, however, show that the lowest energy MLCT band involves the NC-orthometalating ligands rather than the easier to reduce NN-coordinating ligands.Finally, the luminescence spectra show that the lowest energy excited state is predominantly ligand centered (on the NC-orthometalating ligands when NN = bpy or phen and on the NN-coordinating ligands when NN = biq) in nature. These results show that in these complexes there are several closely spaced levels of different orbital nature and confirm that a localized description of their electronic structure can only be an approximation.

2,5‐Bis(p‐R‐arylethynyl)rhodacyclopentadienes Show Intense Fluorescence: Denying the Presence of a Heavy Atom

“…Most organometallic Rh III complexes exhibit no fluorescence, but phosphoresce in a glass at 77 K from predominantly ligand-centered 3 p!p* states with only small 3 MLCT contributions; the lifetimes are generally between t = 0.04-48 ms. [13] Photoluminescence with microseccond lifetimes from metal-centered ligand field states ( 3 LF) has been reported in [Rh(NH 3 ) 5 3+ (trpy = terpyridine). [14] Fluorescence is rarely observed in organometallic and coordination compounds with 4d/5d transition-metal centers, as the (M = Rh, Ir; L = 1,3-diisocyanopropane, 2,5-dimethyl-2,5-diisocyanohexane) undergo excitation to a 1 A 2u (ds*-ps) state.…”

Cyclometalated rhodium(III) complex with phen-dione ligand

“…reduction couple at À2.195 V is assigned to the reduction of the pyridine-2-yl-2-phenyl ligands. As it was pointed out previously, the coordinating N-N ligands (phen or bpy) are much easier to reduce than the orthometalated pyridine-2-yl-2-phenyl ligand [28].…”

“…In this complex, two reversible reduction couples at À0.20 and À0.942 V are assigned to the reduction of phen-dione ligand to phen-semiquinonate and phen-diolate, respectively by analogy to other phen-dione complexes [8][9][10]26,27]. ½Rhðphpy-j 2 N;C 2 0 Þ 2 ðphen-dioneÞ þ þ e À ½Rhðphpy-j 2 N;C 2 0 Þ 2 ðphen-dione À Þ ½Rhðphpy-j 2 N;C 2 0 Þ 2 ðphen-dione À Þ þ e À ½Rhðphpy-j 2 N;C 2 0 Þ 2 ðphen-dione 2À Þ À For the mononuclear ½Mðphpy-j 2 N;C 2 0 Þ 2 ðN À NÞ þ series, reduction of the anionic pyridine-2-yl-2-phenyl ligands usually takes place below À2.3 V [28]. The quasi-reversible Fig.…”

Excited State Spectroscopy and Excited State Dynamics of Rh(III) and Pd(II) Chelates as Studied by Optically Detected Magnetic Resonance Techniques

“…The luminescence observed for the Ir 3+ (5d 6 ) complexes has been assigned as due to a 3 MLCT transition [69]. The Rh 3+ complexes, on the other hand, show exclusively 3 LC luminescence [70,71]. In fact, for the complexes containing the thpyas cyclometalating ligand the excitation is trapped at this ligand.…”